A permanent magnet synchronous motor has been known as a highly efficient motor as compared to an induction motor which has been widely used conventionally in various fields, and an application thereof for driving an automobile or an electric train has been studied in recent years. In an electric vehicle running with a plurality of vehicles, each equipped with such an AC motor and a motor drive control device, being connected together, even if a motor drive control device in part of the vehicles has a failure while running and part of the motors therefore cannot be operated, the electric vehicle can be kept running by other sound AC motor drive control devices and motors. Note however that since the motor connected to the broken AC motor drive control device is kept driven from the wheel side thereof, a short-circuit current due to an induced voltage of the motor is kept flowing through the failure part (short-circuited part) in the AC motor drive control device having a short-circuit fault.
As a method for dealing with a failure in an inverter within an AC motor drive control device for controlling the drive of a permanent magnet synchronous motor while an electric vehicle is running as described above, Patent Literature 1 below, for example, discloses a method in which a motor opening contactor for electrically disconnecting between the inverter and the motor is provided so as to prevent a damage to the inverter due to the induced voltage of the motor from being expanded and if a control unit detects a failure in the inverter, the control unit performs open-circuit control for this contactor so as to electrically disconnect between the inverter and the motor.
Also, as an opening contactor, Patent Literature 2 below mentions a vacuum contactor having a high cutoff performance. Furthermore, Patent Literature 2 discloses a method in which ferrites are disposed on an upstream side and a downstream side of the contactor so as to limit a pre-arc phenomenon or a closing surge voltage between contacts when the vacuum contactor is closed.